Bellaire High School Advanced Physics - Chapter 4

Lesson 4-1Lesson 4-1Changes in MotionChanges in Motion

Causes of Motion ChangeCauses of Motion Change

Force causes a change in motionForce causes a change in motion When we think of a force, usually think of a When we think of a force, usually think of a

push or pullpush or pull Force represents an interaction between an Force represents an interaction between an

object and it environmentobject and it environment

Formal definition of force:Formal definition of force: Causing a change in motionCausing a change in motion

Causes of Motion ChangeCauses of Motion Change

A force may:A force may: Cause a moving object to stopCause a moving object to stop A stationary object to moveA stationary object to move A moving object to change directionA moving object to change direction

The SI unit of force is the NewtonThe SI unit of force is the Newton Named for Sir Isaac NewtonNamed for Sir Isaac Newton

He contributed most of the modern work geared to He contributed most of the modern work geared to understanding forceunderstanding force

The NewtonThe Newton

A Newton (N) is the A Newton (N) is the amount of force amount of force required to accelerate required to accelerate 1 kg of mass 1 m/s1 kg of mass 1 m/s22

1 N = 1 kg · 1 m/s1 N = 1 kg · 1 m/s22

Force is the result of Force is the result of an accelerated massan accelerated mass

Force = mass x Force = mass x accelerationacceleration

F ma

Types of ForceTypes of Force

Contact forcesContact forces Result from physical contact between Result from physical contact between

objectsobjects Stretching a springStretching a spring Pulling a wagonPulling a wagon Catching a ballCatching a ball

This force is very easy to studyThis force is very easy to study The force is easy to see and identifyThe force is easy to see and identify


Field forcesField forces Do not involve direct contactDo not involve direct contact

Examples?Examples? Force from gravityForce from gravity

An object accelerates toward earth, but earth is not An object accelerates toward earth, but earth is not touching the objecttouching the object

The presence of an object affects the space around itThe presence of an object affects the space around it A force is exerted on any other object within that A force is exerted on any other object within that

spacespace The region of influence is called the gravitational fieldThe region of influence is called the gravitational field Objects exert force on each other when their fields Objects exert force on each other when their fields

come into contactcome into contact


Field forcesField forces

Forces from electrical or magnetic fieldsForces from electrical or magnetic fields Force from static electricity may hold a balloon Force from static electricity may hold a balloon

against the wallagainst the wall Forces from magnetic fields attract objectsForces from magnetic fields attract objects

Field forces are extremely important in the study of Field forces are extremely important in the study of particle physics, where particles do not actually touch particle physics, where particles do not actually touch each othereach other

Their interactions are based solely on field forcesTheir interactions are based solely on field forces

Force DiagramsForce Diagrams

If you give a toy car a small push, it will not If you give a toy car a small push, it will not travel as far as if you give it a harder pushtravel as far as if you give it a harder push

The effect of force depends on magnitude and The effect of force depends on magnitude and directiondirection

Force is a…?Force is a…? Vector (magnitude and direction)Vector (magnitude and direction) Force may represented by vectors and added tail to Force may represented by vectors and added tail to

tiptip Force vectors may also be resolved into their Force vectors may also be resolved into their x x and and yy

componentscomponents

Force DiagramsForce Diagrams

Diagrams that show forceDiagrams that show force The tail of the arrow is attached to the object The tail of the arrow is attached to the object

on which the force is actingon which the force is acting At this point, we disregard size and shape of At this point, we disregard size and shape of

the objectthe object We assume the object is one point of mass, We assume the object is one point of mass,

called a “point mass”called a “point mass” We assume all forces acting on the mass act We assume all forces acting on the mass act

on one point, at the center of the object, on one point, at the center of the object, regardless where the forces are appliedregardless where the forces are applied

Free Body DiagramFree Body Diagram

A free body diagram isolates an object A free body diagram isolates an object from its surroundingsfrom its surroundings

Lets say a moving car just ran into a wallLets say a moving car just ran into a wall Draw a free body diagram to represent this Draw a free body diagram to represent this

situationsituation


Shows the forces on one objectShows the forces on one object Useful to find component and resultant Useful to find component and resultant

forcesforces

Recall forces are vectors which may be Recall forces are vectors which may be resolvedresolved


Lets say a tow truck just arrived to take Lets say a tow truck just arrived to take away the car that ran into the wallaway the car that ran into the wall The tow truck is going to hook up to the car The tow truck is going to hook up to the car

and some things are going to happenand some things are going to happen

Lets continue to study the car:Lets continue to study the car: Many forces will act on the car:Many forces will act on the car:

Tow truck cableTow truck cable Force from the road (normal and frictional)Force from the road (normal and frictional) GravitationalGravitational


A FBD will help us sort out these forces and A FBD will help us sort out these forces and allow us to study only the carallow us to study only the car

First, we will draw a very simple car shapeFirst, we will draw a very simple car shape

Lets add the following forces to the carLets add the following forces to the car 5,800 N from the cable5,800 N from the cable 14,700 N gravitational force14,700 N gravitational force 13,690 N road pushing up on the car13,690 N road pushing up on the car 775 N from the interaction between the tires and the road775 N from the interaction between the tires and the road

Lesson 4-2Lesson 4-2Newton’s First LawNewton’s First Law

InertiaInertia

The first step is always to examine the The first step is always to examine the motion of an object before the forces are motion of an object before the forces are appliedapplied Is it moving, stationary… etc?Is it moving, stationary… etc?

Think of pushing a heavy box across a Think of pushing a heavy box across a waxed wooden floor and a carpeted floorwaxed wooden floor and a carpeted floor The box will behave differently because of The box will behave differently because of

different external forcesdifferent external forces

InertiaInertia

An object does not want to move if it is An object does not want to move if it is sitting motionlesssitting motionless

An object does not want to stop if it is in An object does not want to stop if it is in motionmotion

The tendency for an object to resist The tendency for an object to resist change of motion is known as change of motion is known as inertiainertia

InertiaInertia

Say you have a basketball, a medicine ball Say you have a basketball, a medicine ball and a golf ball on the tableand a golf ball on the table Which will be easiest to move?Which will be easiest to move? Which will be most difficult to move?Which will be most difficult to move?

Inertia is an object’s tendency to maintain its Inertia is an object’s tendency to maintain its current statecurrent state

So it would make sense for different objects to So it would make sense for different objects to have different amounts of inertiahave different amounts of inertia

Newton’s First LawNewton’s First Law

The two previous concepts combine to form Newton’s The two previous concepts combine to form Newton’s First LawFirst Law

““An object at rest remains at rest, and an object in An object at rest remains at rest, and an object in motion remains in motion unless it experiences an motion remains in motion unless it experiences an outside force”outside force”

Is often referred to as the law of inertia Is often referred to as the law of inertia When the net external force is 0, an object in motion remains in When the net external force is 0, an object in motion remains in

motion and a stationary object remains motionlessmotion and a stationary object remains motionless Simply, an object seeks its current state and will not change on Simply, an object seeks its current state and will not change on

its ownits own

Net External ForceNet External Force

Consider a car traveling at a constant Consider a car traveling at a constant velocityvelocity Since we have established constant Since we have established constant vv, there , there

is no is no aa and thus no force and thus no force Lets consider a free body diagram of the Lets consider a free body diagram of the

carcar Show and define the following:Show and define the following:

Gravity, Normal force, Force of motion, Gravity, Normal force, Force of motion, Resistant forceResistant force

Summation of ForcesSummation of Forces

Recall the Recall the xx direction vectors are direction vectors are independent of the independent of the yy vectors and the two vectors and the two do not affect each otherdo not affect each other

The NET FORCE is the sum of all of the The NET FORCE is the sum of all of the forces in the forces in the x x and and y y directionsdirections

We denote as Σ FWe denote as Σ Fxx and Σ F and Σ Fyy


Once Σ FOnce Σ Fxx and Σ F and Σ Fyy have been calculated, have been calculated,

a resultant vector can be found, which will a resultant vector can be found, which will be the net force acting on the objectbe the net force acting on the object


If Σ FIf Σ Fxx = 0, there is no net force in the = 0, there is no net force in the xx directiondirection

If Σ FIf Σ Fyy = 0, there is no net force in the = 0, there is no net force in the yy directiondirection

If there is no net force, we say the object is If there is no net force, we say the object is in equilibriumin equilibrium Objects may be in equilibrium only in the Objects may be in equilibrium only in the x x

direction, only in the direction, only in the yy direction, or in both direction, or in both directionsdirections

Column page 132Column page 132 Sample page 132Sample page 132 Practice 133Practice 133

Lesson 4-3Lesson 4-3Newton’s Second and Third Newton’s Second and Third

LawsLaws

Newton’s Second LawNewton’s Second Law

Recall Newton’s first lawRecall Newton’s first law An object with no outside force is in An object with no outside force is in

equilibriumequilibrium We also know an object that experiences an We also know an object that experiences an

outside force undergoes a change in motionoutside force undergoes a change in motion Force is proportional to mass and Force is proportional to mass and

accelerationacceleration Pushing a car by yourself is difficultPushing a car by yourself is difficult

Since there is a force, there will be an accelerationSince there is a force, there will be an acceleration But your force is small, so the accel. is also smallBut your force is small, so the accel. is also small


If you were to push that same car with a If you were to push that same car with a group, it would be easiergroup, it would be easier Since the net force is greater, the accel. is Since the net force is greater, the accel. is

largerlarger Acceleration is directly proportional to the Acceleration is directly proportional to the

net external force acting on itnet external force acting on it Which is why:Which is why:

More force is needed to accel. a large objectMore force is needed to accel. a large object Less force is needed to accel. a small objectLess force is needed to accel. a small object


““The acceleration of an object is directly The acceleration of an object is directly proportional to the net external force proportional to the net external force acting on it and inversely proportional to acting on it and inversely proportional to the mass of the object”the mass of the object”

ΣΣF = maF = ma ΣΣF is the total sum of the vectors forcesF is the total sum of the vectors forces

Resolve all forces into Resolve all forces into xx and and yy components components Calculate Calculate ΣΣFFxx and and ΣΣFFyy

Calculate the resultant vectorCalculate the resultant vector

Example pg 137Example pg 137 Practice pg 138Practice pg 138

Newton’s Third LawNewton’s Third Law

Consider a car colliding with a concrete Consider a car colliding with a concrete barrierbarrier The car exerts a force on the barrier at the The car exerts a force on the barrier at the

moment of the collisionmoment of the collision Similar to pushing a door or kicking a ballSimilar to pushing a door or kicking a ball

The barrier also exerts a force back on the The barrier also exerts a force back on the car, causing the car to rapidly slow downcar, causing the car to rapidly slow down

Pairs of ForcesPairs of Forces

Newton recognized that a single isolated Newton recognized that a single isolated force cannot existforce cannot exist

Forces must exist in pairsForces must exist in pairs This is explained in Newton’s Third LawThis is explained in Newton’s Third Law

Newton’s Third LawNewton’s Third Law

““If two bodies interact, the magnitude of If two bodies interact, the magnitude of the force exerted on object 1 by object 2 is the force exerted on object 1 by object 2 is equal in magnitude but opposite in equal in magnitude but opposite in direction to the force exerted on object 2 direction to the force exerted on object 2 by object 1”by object 1”

Action-Reaction ForcesAction-Reaction Forces

When two objects interact with each other, When two objects interact with each other, they create action-reaction forcesthey create action-reaction forces

It does not matter which force is It does not matter which force is considered the action or which is considered the action or which is considered the reactionconsidered the reaction

Action reaction forces act on different Action reaction forces act on different objectsobjects

For that reason, A/R pairs do not create For that reason, A/R pairs do not create situations of equilibriumsituations of equilibrium


Consider a hammer hitting a nailConsider a hammer hitting a nail The hammer exerts a force on the nail and The hammer exerts a force on the nail and

the nail exerts the same force back on the the nail exerts the same force back on the hammerhammer

So why doesn’t the nail stay put?So why doesn’t the nail stay put? A free body diagram shows us why.A free body diagram shows us why.


Field forces also exist in A/R pairsField forces also exist in A/R pairs Lets say we drop a bowling ball from the top of the Lets say we drop a bowling ball from the top of the

buildingbuilding During the collision with the ground, the ball exerts a During the collision with the ground, the ball exerts a

force on the ground and the ground exerts a force on force on the ground and the ground exerts a force on the ballthe ball

But, during the fall, the force from gravity also exists But, during the fall, the force from gravity also exists in a A/R pairin a A/R pair The ball falls toward the Earth AND the Earth also “falls” The ball falls toward the Earth AND the Earth also “falls”

toward the balltoward the ball To make sense of this, recall that acceleration is inversely To make sense of this, recall that acceleration is inversely

proportionate to the size of the massproportionate to the size of the mass Since the ball is so small in relation to the Earth, the ball’s Since the ball is so small in relation to the Earth, the ball’s

acceleration is much more noticeable than the Earth’sacceleration is much more noticeable than the Earth’s

Section Review pg 140Section Review pg 140

Lesson 4-4Lesson 4-4Everyday ForcesEveryday Forces

WeightWeight

Why does a tennis ball feel lighter than a Why does a tennis ball feel lighter than a bowling ball?bowling ball? The weight of an object is defined as the The weight of an object is defined as the

magnitude of the force of gravity on that magnitude of the force of gravity on that objectobject

Recall F=maRecall F=ma Weight is a direct result of the force exerted Weight is a direct result of the force exerted

by the Earth on an objectby the Earth on an object

WeightWeight

Is a vector quantity directed to the center Is a vector quantity directed to the center of the Earthof the Earth Straight downStraight down

FFgg=ma=ma But since g is an accel… FBut since g is an accel… Fgg=mg=mg

Also depends on locationAlso depends on location You would weigh a lot less on the moon since You would weigh a lot less on the moon since

ggmoonmoon < g < gEarthEarth

Normal ForceNormal Force

You are sitting in a seat, right nowYou are sitting in a seat, right now Why are you not falling on the floor?Why are you not falling on the floor?

You are subject to Earth’s gravity, so shouldn’t you You are subject to Earth’s gravity, so shouldn’t you keep accelerating to the center of the Earth?keep accelerating to the center of the Earth?

You exert a force on you chair, but the chair also You exert a force on you chair, but the chair also exerts an equal and opposite force back on youexerts an equal and opposite force back on you

This is a special case of A/R forces where This is a special case of A/R forces where equilibrium exists (Remember, this is not always equilibrium exists (Remember, this is not always the case)the case)

Normal ForceNormal Force

The force that is pushing back on you from The force that is pushing back on you from your seat is the normal forceyour seat is the normal force Symbolized by FSymbolized by FNN

The FThe FNN is always perpendicular to the is always perpendicular to the

surfacesurface Even though the weight of the object is Even though the weight of the object is

always directed straight downalways directed straight down

Ex: Box on the floor, box on a rampEx: Box on the floor, box on a ramp

Force of FrictionForce of Friction

Consider an object on a tableConsider an object on a table Newton’s first law tells us this object is in Newton’s first law tells us this object is in

equilibriumequilibrium Newton’s second law tells us this object will Newton’s second law tells us this object will

accelerate if an outside force is appliedaccelerate if an outside force is applied The first law says the object will stay in motion until The first law says the object will stay in motion until

acted on by an outside forceacted on by an outside force

Force of FrictionForce of Friction

But experience tells us that even a small But experience tells us that even a small force will not move the objectforce will not move the object

And if it does move, it will stop almost And if it does move, it will stop almost immediately immediately

WHY?WHY?

Friction Opposes the Applied Friction Opposes the Applied ForceForce

A small force is not enough to overcome A small force is not enough to overcome the frictional forcethe frictional force The frictional force that keeps objects from The frictional force that keeps objects from

moving is called moving is called static friction (Fstatic friction (Fss))

A large force may start the object moving, A large force may start the object moving, but it eventually slows and stopsbut it eventually slows and stops The frictional force that slows and stops The frictional force that slows and stops

objects is called objects is called kinetic friction (Fkinetic friction (Fkk))

Two Types of FrictionTwo Types of Friction

Static friction, FStatic friction, Fss, is always greater than kinetic , is always greater than kinetic

friction, Ffriction, Fkk

It is always harder to get something moving than it is It is always harder to get something moving than it is to keep it movingto keep it moving

More scientifically, the molecules of the two More scientifically, the molecules of the two surfaces in contact bondsurfaces in contact bond

To move an object, you must break these bondsTo move an object, you must break these bonds Once the bonds are broken and the object begins to Once the bonds are broken and the object begins to

move, is it easy to keep it movingmove, is it easy to keep it moving

Surface MattersSurface Matters

Friction depends on the surfaces in Friction depends on the surfaces in contactcontact No surface is perfectly smoothNo surface is perfectly smooth Surfaces are just less rough than othersSurfaces are just less rough than others

For instance a sled moves easier over icy snow For instance a sled moves easier over icy snow than over a paved roadthan over a paved road

Friction is proportionate to the normal forceFriction is proportionate to the normal force The normal force of an object is the same over The normal force of an object is the same over

different surfacesdifferent surfaces The difference is the make-up of the surfaceThe difference is the make-up of the surface

Surface MattersSurface Matters

To express the relationship between To express the relationship between frictional forces and the properties of frictional forces and the properties of different surfaces we use the Greek different surfaces we use the Greek symbol mu (symbol mu (μμ)) μμ is the coefficient of friction and is used to is the coefficient of friction and is used to

find the value of the frictional force between find the value of the frictional force between two surfacestwo surfaces FFss= = μμssFFnn

FFkk= = μμkkFFnn μμ changes depending on what two surfaces are in changes depending on what two surfaces are in

contactcontact

A Ratio of ForcesA Ratio of Forces

The coefficient of friction is a ratio of The coefficient of friction is a ratio of forcesforces Experiments have shown that the value for Experiments have shown that the value for μμ

may range from .001 to 1.5 between naturally may range from .001 to 1.5 between naturally occurring materialsoccurring materials

A A μμ value of 1.00 indicates you would need a value of 1.00 indicates you would need a force equal to the normal force to move the force equal to the normal force to move the objectobject

If If μμ were 1.5, you would need a force 1.5 were 1.5, you would need a force 1.5 times the normal force to move the objecttimes the normal force to move the object

Extreme Extreme μμ’s’s

Smallest Smallest μμ Ice at 32Ice at 32oo sliding over ice at 32 sliding over ice at 32oo

Human jointsHuman joints

Largest Largest μμ Synthetic materialsSynthetic materials

Most notably: receiver gloves (Most notably: receiver gloves (μμ’s around 4.00)’s around 4.00)

Air ResistanceAir Resistance

Air Resistance is a form of frictionAir Resistance is a form of friction Opposes the applied force and retards Opposes the applied force and retards

progressprogress FFRR is usually proportionate to speed is usually proportionate to speed

Higher speeds = Larger Air ResistancesHigher speeds = Larger Air Resistances There exists terminal velocity, which is most There exists terminal velocity, which is most

notable in free fallnotable in free fall FFRR is equal to F is equal to Fgg

Creates equilibrium, no accelerationCreates equilibrium, no acceleration Object continues to fall at a constant velocityObject continues to fall at a constant velocity

Example pg 145Example pg 145 Practice pg 145Practice pg 145 Example pg146Example pg146 Practice pg 147Practice pg 147

Education

Bellaire High School Advanced Physics - Chapter 4